1. Field of the Invention
This invention relates to a method of producing printed circuit boards.
2. Related Art Statement
In general, the printed circuit board is constructed by forming a given circuit pattern on an insulation substrate through an electric conductor and assembling a given electronic part on the circuit pattern.
The assembling of the electronic part on the printed circuit board is carried out by immersing the circuit pattern surface of the printed circuit board in a fused solder bath or a jet type solder bath at a state of mounting the electronic part on the printed circuit board to attach solder to the circuit pattern surface to thereby electrically and mechanically connect leads of the electronic part to the circuit pattern.
In the soldering between the circuit pattern and the lead of the electronic part, a solder resist is previously applied to the whole surface of the circuit pattern other than land portions to be soldered in order to attach the solder to only the connecting portions therebetween. However, the distance between the adjoining circuits becomes narrower with the miniaturization of the circuit pattern and hence the distance between the lead of the electronic part and the connecting land becomes also narrower, whereby the function inherent to the solder resist is damaged to caused bridging phenomenon between mutual lands due to the soldering. Therefore, it is frequently required to conduct the correction operation and the like.
In order to prevent such a bridging of solder, particularly the bridging of solder at a position of narrow land distance, there is adopted a method, wherein a conductor pattern is printed on an insulation substrate, and a first soldered resistor layer is formed over the whole surface of the conductor pattern so as to leave lands to be soldered, and a second soldered resistor layer for preventing the bridging of solder on at least positions of narrow land distance is formed on the first soldered resistor layer in the production of the printed circuit board. This method has been disclosed in Japanese Patent Application Publication No. 54-41162.
However, when the second soldered resistor layer for the prevention of the bridging is formed on the first soldered resistor layer, since the first soldered resistor layer is previously formed over the whole surface of the conductor pattern so as to leave lands to be soldered, it is required to have a high aligning accuracy for leaving the lands to be soldered. Further, since the second soldered resistor layer for the prevention of the bridging is formed on the first soldered resistor layer after the formation of the first soldered resistor layer on the conductor pattern, the solder resist ink through screen printing for the formation of the second soldered resistor layer is undesirably oozed out on the land portions to be soldered in addition to the above aligning accuracy.